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What are peptides?

Short chains of amino acids that act as the body's signalling molecules. How they work, how they're given, and the families you'll actually hear about.

A simple definition

A peptide is a short chain of amino acids — the same building blocks as protein — usually 2 to 50 of them linked by peptide bonds. Longer chains get called proteins. Your body already makes thousands and uses them as messengers: hormones, neurotransmitters, growth factors, immune signals.

Each peptide fits a specific receptor like a key fits one lock, which is why tiny doses can produce targeted effects. Not new science either — insulin, the first peptide medicine, has been in use since 1922. Semaglutide runs on the same key-and-lock principle a century later.

How they work in the body

Most therapeutic peptides are receptor agonists — they mimic or amplify a signal the body already sends. Four mechanisms cover nearly everything in this handbook:

• Incretin mimics (semaglutide, tirzepatide, retatrutide) — copy gut hormones that curb appetite, slow stomach emptying and improve insulin response. The engine of the weight-loss boom.
• Growth-hormone secretagogues (sermorelin, CJC-1295, ipamorelin, tesamorelin) — nudge the pituitary to release its own growth hormone rather than injecting GH directly.
• Regenerative peptides (BPC-157, TB-500) — proposed to influence blood-vessel growth, cell migration and tissue repair. Proposed is the operative word: human evidence is close to non-existent.
• Melanocortin agonists (PT-141, melanotan) — act on brain pathways for sexual desire and on skin pigmentation.

Why they're injected

Peptides are fragile. Swallowed, gut enzymes shred most of them before they act — so nearly all are given by subcutaneous injection into the fat under the skin. Exceptions: a few nasal sprays, and non-peptide mimics like MK-677 that survive as tablets. The same fragility is why storage and preparation matter so much.

The main families

Learn

Uses & the honest evidence

Class by class: what it's used for, and how strong the human evidence really is. Badges: Approved licensed medicine · Strong good human trials · Moderate some human data · Weak little or indirect · Research only essentially none.

Side effects by class

Learn

Research & clinical case studies

Landmark trials in plain English — who was studied, what happened, how strong it is. The gap between what's claimed and what's been proven in humans is the whole story of this space.

References

• Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021. doi:10.1056/NEJMoa2032183
• Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med 2022. doi:10.1056/NEJMoa2206038
• Tesamorelin in people with HIV on integrase inhibitors. AIDS 2024. PMC11365754
• Jastreboff AM et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity (Phase 2). N Engl J Med 2023. doi:10.1056/NEJMoa2301972
• Eli Lilly. Retatrutide TRIUMPH Phase 3 topline results, Dec 2025 (investor release)
• Pickart L, Margolina A. Skin Regenerative and Anti-Cancer Actions of Copper Peptides. Cosmetics 2018 (MDPI)
• BPC-157 human trials — systematic review summary, 2025 (peptide-db)
• Nass R et al. Ghrelin mimetic MK-677 in healthy older adults. Ann Intern Med 2008

Where a journal link is paywalled, the DOI locates the paper. Secondary sources only summarise underlying trials.

Learn

Fifteen facts worth knowing

The science, the market and the safety realities — fast orientation, sourced. Split by theme.

The science

Medicine & the market

Safety & the grey market

Expanded with citations across the handbook: Research for the evidence · Statistics for the numbers · Dosing for the units · Check a COA for the safety points.

Learn

The boom, in numbers

Market size, GLP-1 sales, how common use has become, and the state of the evidence. Every figure sourced; the audited sales are the hard numbers, projections are direction only.

GLP-1 drug sales, full-year 2024

The market, and its uncertainty

Peptide-therapeutics market projections vary wildly — roughly $87bn to $295bn by the early-to-mid 2030s depending on definition. Treat as direction, not precision. What's not in doubt: North America held ~60% of the market in 2024, and peptide contract manufacturing is forecast to hit ~$5.7bn by 2030 (~12.5%/yr), driven overwhelmingly by GLP-1 demand.

Directory

56 peptides, rated honestly

Every compound you're likely to hear about — profile, evidence badge, commonly-reported dosing where one circulates, and one safety note. Inclusion is never a recommendation.

Profiles compiled from regulator announcements (MHRA, FDA, EMA), peer-reviewed trials and company releases, current to mid-2026. Fast-moving field — verify anything important against primary sources.

Directory

Compare any two

Category, mechanism, evidence, reported dosing and safety, side by side. Popular pairs get full write-ups on Head-to-heads.

Dosing

Dosing, injecting & cycling

Units, kit, routes, sites, technique, timing, stacking and cycling — the complete practical picture.

Units: the 1,000× trap

1 milligram (mg) = 1,000 micrograms (mcg). GLP-1 medicines are dosed in milligrams weekly; many GH and research peptides are quoted in micrograms. Mixing the units is a thousand-fold error — the entire distance between a normal amount and a dangerous overdose lives in one letter. Check the unit before anything else, every time.

Storing peptides

• Middle fridge shelf beats the door — fewer temperature swings.
• Brief room-temperature transit is usually tolerated; refrigerate on arrival.
• Cloudy, discoloured, or particles = bin it. No exceptions.

Needles & kit

Two jobs, two needles: moving liquid (thicker, faster) and injecting (as fine as possible). Gauge = thickness; higher number = thinner.

Plus alcohol swabs, bacteriostatic water, clean hands, and a sharps bin. Needles are single-use — reuse blunts them, sharing risks bloodborne infection, neither is negotiable.

Subcutaneous vs intramuscular

Subcutaneous (subQ) = into the fat under the skin with a short fine needle — the route for GLP-1 medicines, tesamorelin, and nearly everything else here. Intramuscular goes deeper, absorbs differently, and is uncommon for these compounds. Everything below is the subQ route.

Injection sites & rotation

Safe technique, step by step

Standard subcutaneous technique — same method as insulin. First-timers should have a nurse or pharmacist demonstrate. That's the best move, not a disclaimer.

1. Wash hands properly — soap, water, no shortcuts.
2. Check the solution: clear, particle-free, in date. Confirm the amount and the unit.
3. Swab the vial stopper and the skin; let both air-dry.
4. Draw the dose, flick out air bubbles, expel the air.
5. Pinch a fold of skin to lift fat away from muscle.
6. Insert in one smooth motion — 90° for short insulin needles, 45° if very lean.
7. Inject slowly and steadily; release the pinch.
8. Withdraw at the same angle; light pressure with clean gauze — don't rub.
9. Needle straight into the sharps bin. Never household waste, never recapped.
10. Log it: site, amount, batch number, time.

Timing, by class

Stacking — the honest picture

"Stacking" = running multiple compounds at once — the classic CJC-1295 + ipamorelin pairing, or BPC-157 + TB-500. People stack for theoretical synergy; what it actually multiplies is unknowns: more side-effect sources, more interactions, more contamination exposure, and no way to attribute anything.

• One at a time. Run a single compound 1–2 weeks before adding anything, so reactions can be traced.
• Don't mix unknowns in one syringe unless compatibility is actually established.
• Mind total load — more compounds = more injections = more sterility risk.
• Short and simple beats long and experimental — and a clinician should steer any combination.

Cycling & receptor tolerance

A period on, a period off. Rationale: receptor desensitisation — hammer a receptor continuously and it responds less, so a break is thought to reset it. Sound pharmacology; but the specific schedules quoted online (the classic "12 on, 4 off" for GH secretagogues) are community conventions. Nobody has trialled them.

Licensed schedules (reference)

Dosing

Reconstitution & dose reference chart

The commonly reported figures for 25+ compounds in one place — a typical vial, the water people add, the concentration that results, the syringe units that equals, and the dose/frequency/cycle patterns you'll see quoted.

* Units on a U-100 insulin syringe (100 u = 1 mL), valid only for the example mix in the same row. † A 200–600 mg glutathione amount exceeds one insulin syringe at this concentration — exactly what the calculator exists to catch. "Of each" in blends = one injection delivers that amount of every component simultaneously. Compiled from vendor sheets and community sources, cross-checked against licensed product information where one exists; for unlicensed compounds these are not clinically validated.

Dosing

How to reconstitute a peptide

Peptides arrive as freeze-dried powder that has to be dissolved before it can be measured. The standard method, the five moves, and the maths.

What reconstitution is

Peptides ship lyophilised — freeze-dried into a white pellet in a sealed vial, stable while dry. Reconstitution just means adding a sterile liquid (a diluent) so it becomes a measurable solution.

The standard diluent is bacteriostatic water — sterile water with ~0.9% benzyl alcohol, a preservative that suppresses microbial growth and keeps a mixed vial usable ~4 weeks refrigerated. Plain sterile water has no preservative: a vial mixed with it is vulnerable within hours. The end product is injected, so sterility is the whole game — contamination means infection, abscess or systemic reaction, and bacterial endotoxin is heat-stable and invisible to the HPLC purity tests vendors publish.

The kit

• Vial of lyophilised peptide
• Bacteriostatic water
• Alcohol swabs — for both rubber stoppers
• Sterile single-use syringe to move the water
• Clean, flat surface

The five moves

1. Swab both stoppers. Fresh alcohol wipe on both vials' rubber tops; air-dry. The most commonly cut corner and the likeliest contamination point.
2. Draw the bacteriostatic water. The amount is a concentration choice, not a fixed rule — more water = weaker, easier-to-measure solution. Calculator below.
3. Run it down the glass, slowly. Tilt the peptide vial, let water slide down the inside wall. Never jet it onto the powder — shear force can damage the peptide. The step that matters most.
4. Swirl, never shake. Most peptides dissolve in a minute or two; roll gently if needed. Shaking foams and can denature. Result should be completely clear.
5. Refrigerate, labelled with the date. 2–8°C, out of light, ~4 weeks with bacteriostatic water. Never freeze a mixed vial.

The calculator

U-100 insulin syringe: 100 units = 1 mL. Concentration = peptide ÷ water. Worked arithmetic — never carry a unit figure from any chart to a different vial.

Storage & when to discard

Mixed vials: refrigerated, dark, dated. Discard if cloudy, particulate, past the preservative window, or stored warm. When in doubt, throw it out.

Dosing

Microdosing vs one larger dose

The buzziest dosing idea going. The theory is real pharmacology; the evidence it delivers better outcomes is thin to absent.

Two meanings circulate: splitting the same total into frequent small injections, or deliberately running below standard doses (common with GLP-1s to save money or dodge nausea). This page compares frequent-small vs one-larger.

Where the theory holds

Split the same amount into smaller, more frequent doses and blood levels sit flatter — trading one peak-and-crash for a gentler line. For GH peptides there's extra logic: GH releases in pulses, so frequent small doses arguably mimic the rhythm (exactly why short-acting Mod GRF 1-29 and ipamorelin get dosed several times daily). Many side effects track the peak, so a lower peak could mean a smoother ride.

Where it runs into reality

• Side effects can just return more often. Smaller doses clear faster — re-dose and the same effect can come straight back rather than being avoided.
• More injections = more risk events. Every extra jab is another site reaction, infection or technique-slip opportunity, and tiny part-draws are imprecise plus another contamination chance each time.
• For licensed GLP-1s the studied schedule is standard titration under a prescriber. No clinical body endorses DIY microdosing, and no evidence shows it matches the trial results.
• It does nothing about the vial. Microdosing an unlicensed peptide makes it no purer and no better labelled. Sourcing risk identical at any injection frequency.

Bottom line: pharmacokinetic theory, cost and anecdote drive this choice — not outcome data. Pair with the chart, Check a COA and Responsible use.

Stay safe

Responsible use, bloodwork & health checks

What careful, medically supervised use looks like — the checks, the monitoring rhythm, the stop-signs. Read this page before acting on anything else in the handbook.

The golden rules

1. Clinician first — ideally one who knows the specific compound; full honesty about everything being taken.
2. Baseline bloods before anything — so change is measured against the person's own normal, not a guess.
3. One change at a time — three compounds at once means never knowing which helped or hurt.
4. Lowest effective amount — most side effects scale with dose; more is not better.
5. Verify the product — independent COA, ideally own-sample testing. See Check a COA.
6. Sterile technique, every time — see Reconstitution.
7. Track everything — amounts, timing, batch numbers, symptoms, blood trends.
8. Know when to stop — have the red-flag list before it's needed.

Baseline bloodwork

A sensible baseline catches problems that make peptides a bad idea in the first place — undiagnosed diabetes, kidney or liver issues, cardiac risk. Compound- and person-dependent; a broad panel commonly includes:

Ongoing monitoring

Not one-and-done. Sensible rhythm: baseline → re-test 6–8 weeks → every ~3 months while continuing, plus whenever dose changes or something feels off. Bring the clinician the trend, not one number.

Red flags — stop and seek care

• Severe allergic reaction — face/throat swelling, breathing difficulty, widespread rash: emergency, 999.
• Injection-site infection — spreading redness, heat, swelling, pus, fever.
• Severe or persistent abdominal pain (possible pancreatitis, especially on GLP-1s), or pain boring to the back.
• Chest pain, palpitations, fainting, persistently racing heart.
• Sudden vision changes, severe headache, new neurological symptoms.
• Low blood sugar signs — shaking, sweating, confusion; higher risk alongside insulin or sulfonylureas.
• Any new or changing mole or lesion — especially around melanotan-type products.
• Persistent vomiting, jaundice, dark urine.

Who shouldn't go near this

Generally inadvisable — or specialist-only — for anyone pregnant, trying to conceive, or breastfeeding; with current or past cancer (growth-promoting compounds especially); with uncontrolled diabetes, heart, liver or kidney disease; with personal/family medullary thyroid cancer or MEN2 history (hard contraindication for GLP-1s); or competing in tested sport. Interactions count: GLP-1s + insulin or sulfonylureas raise hypoglycaemia risk; any injectable adds bleeding considerations on anticoagulants.

Getting checks in the UK

GP first if there are symptoms or underlying conditions. Regulated private blood-testing services cover baseline and hormone panels without referral. Either way: whoever interprets the results needs the full picture of what's actually being taken.

Stay safe

The grey market & bogus sellers

Why an unregulated "research chemical" market exists, what's actually been found in vials, and how to tell merely-risky from outright scam.

What it is: unapproved peptides can't legally be sold for human use in the UK or US — so they're sold anyway, labelled "for research use only", by vendors whose marketing winks at the opposite. Regulators call the label a legal fiction. Enforcement is climbing: the FDA issued 50+ warning letters at peptide sellers in September 2025 alone. Demand (social media + the GLP-1 wave) met cheap overseas manufacturing and a regulatory gap — a fast market with no built-in quality guarantee.

What's actually been found in vials

Lessons from community scam-tracking

• Contaminants purity tests never look for. Community screening has returned preliminary positives for dangerous adulterants — including fentanyl — in grey-market vials. "99% pure" by HPLC was never designed to catch that.
• Labs only test what's paid for. No contaminant line on a COA ≠ passed — usually means nobody checked. Screening often happens only after a scare.
• Same powder, many brands. Much grey stock is hand-relabelled from a few source manufacturers — a "trusted brand" may be the same batch in new packaging.
• Sellers vanish and rebrand. Unanswered messages, deleted groups, surprise substitute vials, sudden price hikes — the standard exit before a rebrand.

Field reporting distilled from community scam-tracking (K-Hole / @Krysia830073) plus FDA warning letters, TGA and Health Canada notices, and community lab investigations. Treat preliminary community findings as reason for caution pending confirmation — but don't dismiss credible safety warnings while waiting.

Stay safe

How to check a COA is real

A Certificate of Analysis is only as good as its source. What one should contain, how fakes give themselves away, and how to test a product independently.

What a COA should contain

Supplier, the independent testing lab, product and batch/lot number, results, and a date. The tests to expect:

Spotting a fake

• Verify on the lab's own portal. Reputable labs give each report a unique key or QR checked on their site. A forger can fake a PDF, not a record in the lab's database.
• A named, independent lab. A certificate wearing only the seller's branding is self-made and worthless.
• Check the client field. Different company named? Borrowed COA describing someone else's product.
• Match batch number and date on the physical vial to the certificate. A COA validates one batch, ever.
• Fabrication tells: "100.00%" purity, perfectly symmetrical peaks with no baseline noise, identical COAs across products, dead verification links, mismatched dates.

Labs the community actually uses

Aggregators (Finnrick, VialAudit, PeptideBenchmark) collate results but run no assays — context, not proof. Posting a sample for HPLC + identity (+ endotoxin at fuller labs) sidesteps vendor cherry-picking; typical turnaround 1–2 weeks; contact the lab first for sample size and submission steps.

Stay safe

Who actually makes peptides

From the pharma giants behind the GLP-1s to contract manufacturers, compounding pharmacies, and the independent labs vetting the grey market.

The pharmaceutical originators

Novo Nordisk (Denmark) makes semaglutide — Ozempic, Wegovy, Rybelsus — a franchise worth ~$31bn in 2024. Eli Lilly (US) makes tirzepatide (Mounjaro, Zepbound) with ~$45bn total 2024 revenue driven by it. Full GMP manufacturing, full regulatory oversight — the standard the grey market cosplays.

Contract manufacturers (CDMOs)

Most peptide active ingredient is actually made by specialist contract manufacturers, and GLP-1 demand triggered a historic capacity build-out:

• Bachem (Switzerland) — leading pure-play peptide CDMO, expanding Swiss + California sites.
• PolyPeptide Group (Switzerland) — six GMP sites since 1952; agreed a ~$1.8bn acquisition by Samsung Biologics in July 2026, the largest Korean biopharma deal to date.
• CordenPharma — investing €1bn+ into peptide capacity.
• Plus SK Pharmteco, Almac, Axplora, Neuland Laboratories.

Compounding pharmacies (503A vs 503B)

In the US, 503A pharmacies compound patient-specific medicines for an individual with a documented clinical need; 503B outsourcing facilities compound at scale under stricter, GMP-style FDA oversight. During the 2022–25 GLP-1 shortages, compounded semaglutide and tirzepatide were widely and legally available — but the FDA declared the shortages resolved (tirzepatide Dec 2024, semaglutide Feb 2025) and has moved to exclude these drugs from large-scale compounding.

Independent testing labs

The community's quality backstop — Janoshik Analytical (Prague) is most used: HPLC purity, mass-spec identity, endotoxin, metals and sterility with QR-verifiable certificates. Key caveat: not ISO 17025 accredited, results unsuitable for regulatory submission — a community vetting tool. Accredited alternatives: Vanguard Laboratory, MZ Biolabs. Full table on Check a COA.

Stay safe

Peptides & UK law

How peptides are regulated in the UK — licensed medicines, the "research use only" zone, selling and importing, and anti-doping. General information current to mid-2026, not legal advice.

The short version: medicines are regulated by the MHRA under the Human Medicines Regulations 2012. A peptide's legal position depends on what it is and how it's supplied: a few are licensed medicines, most are sold as "research chemicals", and selling any of them for human use without a licence is unlawful.

The clear-cut example: melanotan

The MHRA has repeatedly warned that Melanotan I and II tanning injections and nasal sprays are unlicensed medicines, illegal to sell in the UK, and unsafe. The general rule in one product: no licence + sold for human use = unlawful, however it's marketed.

Anti-doping

Separately from criminal law: GH secretagogues, IGF-1, TB-500 and similar are explicitly on the WADA Prohibited List, and unapproved compounds like BPC-157 are caught by the non-approved-substances rule. Anyone in tested sport treats every peptide in this handbook as bannable.

Sources: Human Medicines Regulations 2012 (legislation.gov.uk) · MHRA warnings on melanotan/unlicensed injectables (gov.uk) · UK Anti-Doping prohibited list (ukad.org.uk).

Reference

Guides by goal

The eight things people actually ask about, with the honest verdict on each. Evidence-first, never a recommendation.

Reference

The big head-to-heads

Seven comparisons that come up constantly — table first, verdict after. Build your own on Compare.

Reference

Glossary

Plain-English definitions for every term in this handbook — agonist to WADA. Type to filter.

Reference

Straight answers

The questions people actually ask, answered without hedging. Education, not medical or legal advice.